Characterization of microsome-associated tobacco BY-2 centrins

Arabidopsis TONNEAU1 proteins are essential for preprophase band formation and interact with centrin

Plant cells have specific microtubule structures involved in cell division and elongation. The tonneau1 (ton1) mutant of Arabidopsis thaliana displays drastic defects in morphogenesis, positioning of division planes, and cellular organization. These are primarily caused by dysfunction of the cortical cytoskeleton and absence of the preprophase band of microtubules. Characterization of the ton1 insertional mutant reveals complex chromosomal rearrangements leading to simultaneous disruption of two highly similar genes in tandem, TON1a and TON1b. TON1 proteins are conserved in land plants and share sequence motifs with human centrosomal proteins. The TON1 protein associates with soluble and microsomal fractions of Arabidopsis cells, and a green fluorescent protein-TON1 fusion labels cortical cytoskeletal structures, including the preprophase band and the interphase cortical array. A yeast two-hybrid screen identified Arabidopsis centrin as a potential TON1 partner. This interaction was confirmed both in vitro and in plant cells. The similarity of TON1 with centrosomal proteins and its interaction with centrin, another key component of microtubule organizing centers, suggests that functions involved in the organization of microtubule arrays by the centrosome were conserved across the evolutionary divergence between plants and animals.

The spectral studies on the effect of Glu 101 to the metal binding characteristic of Euplotes octocarinatus centrin

Glu is highly conserved as the first amino acid of E-helix of the EF-hand protein. In this paper, Glu 101, the first amino acid of E-helix of the third EF-hand motif in Euplotes octocarinatus centrin (EoCen) was mutated to be Lys by the method of site direct mutation. Tb3+ and TNS were used as fluorescence probes in the study of the effect of this mutation to the metal binding characteristic of EoCen by fluorescence spectra. Results indicate that compared with EoCen, the mutation protein (E101K) displays a different Tb3+ binding characteristic and an increased hydrophobic exposure surface. Polyacrylamide gels electrophoresis indicated that the electrophoretic mobilities of EoCen and E101K are distinctly different. It can be deduced that the conformation of EoCen has been altered by this mutation. The general conditional binding constant of Tb3+ to the three loops of EF-hand sites I-III in E101K was calculated to be (5.64+/-0.57)x10(5)M(-1) according to the modified equation of the single binding process.

Cytoskeleton and morphogenesis in brown algae

BACKGROUND

Morphogenesis on a cellular level includes processes in which cytoskeleton and cell wall expansion are strongly involved. In brown algal zygotes, microtubules (MTs) and actin filaments (AFs) participate in polarity axis fixation, cell division and tip growth. Brown algal vegetative cells lack a cortical MT cytoskeleton, and are characterized by centriole-bearing centrosomes, which function as microtubule organizing centres.

SCOPE

Extensive electron microscope and immunofluorescence studies of MT organization in different types of brown algal cells have shown that MTs constitute a major cytoskeletal component, indispensable for cell morphogenesis. Apart from participating in mitosis and cytokinesis, they are also involved in the expression and maintenance of polarity of particular cell types. Disruption of MTs after Nocodazole treatment inhibits cell growth, causing bulging and/or bending of apical cells, thickening of the tip cell wall, and affecting the nuclear positioning. Staining of F-actin using Rhodamine-Phalloidin, revealed a rich network consisting of perinuclear, endoplasmic and cortical AFs. AFs participate in mitosis by the organization of an F-actin spindle and in cytokinesis by an F-actin disc. They are also involved in the maintenance of polarity of apical cells, as well as in lateral branch initiation. The cortical system of AFs was found related to the orientation of cellulose microfibrils (MFs), and therefore to cell wall morphogenesis. This is expressed by the coincidence in the orientation between cortical AFs and the depositing MFs. Treatment with cytochalasin B inhibits mitosis and cytokinesis, as well as tip growth of apical cells, and causes abnormal deposition of MFs.

CONCLUSIONS

Both the cytoskeletal elements studied so far, i.e. MTs and AFs are implicated in brown algal cell morphogenesis, expressed in their relationship with cell wall morphogenesis, polarization, spindle organization and cytokinetic mechanism. The novelty is the role of AFs and their possible co-operation with MTs.

CENTRIN2 modulates homologous recombination and nucleotide excision repair in Arabidopsis

A genetic screen of a population of Arabidopsis thaliana lines exhibiting enhanced somatic homologous recombination yielded a mutant affected in expression of a gene encoding a caltractin-like protein (centrin). The hyperrecombinogenic phenotype could be reproduced using RNA interference (RNAi) technology. Both the original mutant and the RNAi plants exhibited a moderate UV-C sensitivity as well as a reduced efficiency of in vitro repair of UV-damaged DNA. Transcription profiling of the mutant showed that expression of components of the nucleotide excision repair (NER) pathway and of factors involved in other DNA repair processes were significantly changed. Our data suggest an indirect involvement of centrin in recombinational DNA repair via the modulation of the NER pathway. These findings thus point to a novel interconnection between an early step of NER and homologous recombination, which may play a critical role in plant DNA repair.

Gamma-tubulin distribution during cortical microtubule reorganization at the M/G1 interface in tobacco BY-2 cells

Cortical microtubules are considered to regulate the direction of cellulose microfibril deposition. Despite their significant role in determining cell morphology, cortical microtubules completely disappear from the cell cortex during M phase and become reorganized at G1 phase. The mechanism by which these microtubules become properly formed again is, however, still unclear. We have proposed that the origin of cortical microtubules is on the daughter nuclear surface, but further cortical microtubule reorganization occurs at the cell cortex. Hence it is probable that the locations of microtubule organizing centers (MTOCs) are actively changing. However, the actual MTOC sites of cortical microtubules were not clearly determined. In this paper, we have examined the distribution of gamma-tubulin, one of the key molecules of MTOCs in various organisms, during cortical microtubule reorganization using both immunofluorescence and a GFP reporter system. Using a monoclonal antibody (clone G9) that recognizes highly conserved residues in y-tubulin, y-tubulin was found to be constitutively expressed and to be clearly localized to microtubule structures, such as the preprophase bands, spindles, and phragmoplasts, specific to each cell cycle stage. This distribution pattern was confirmed by the GFP reporter system. During cortical microtubule reorganization at the M to G1 transition phase, gamma-tubulin first accumulated at the daughter nuclear surfaces, and then seemed to spread onto the cell cortex along with microtubules elongating from the daughter nuclei. Based on the results, it was confirmed that daughter nuclear surfaces acted as origins of cortical microtubules, and that further reorganization occurred on the cell cortex.

A gamma-tubulin antibody against a plant peptide sequence localises to cell division-specific microtubule arrays and organelles in plants

Gamma tubulin (gamma-tubulin) is involved in microtubule initiation in the eukaryotes. In animal cells it is localised to centrosomes and to other, non-centrosomal sites of microtubule initiation. In addition, cytoplasmic complexes containing gamma-tubulin (gamma-TuRCs; gamma-somes) have been described, which are multiprotein complexes involved in microtubule initiation. Most localisations of gamma-tubulin in plants have previously been achieved using an antibody directed towards a conserved peptide sequence found in animal cells, showing co-localisation with all microtubule arrays throughout the cell cycle. Because different antibodies may give various patterns of subcellular localisation, in the present study we raised a polyclonal antibody ('Hayley') to the plant peptide sequence EDFATQGGDRKDVFFY (bold letters indicate plant-specific amino acids) to further investigate the subcellular distribution in plants. Immunoblotting using wheat root tip protein extracts revealed a 58 kDagamma-tubulin-like peptide as has been described before. Immunofluorescence microscopy of wheat root-tip cells, however, revealed localisation of gamma-tubulin to a subset of mitotic microtubule arrays and the cytokinetic phragmoplast, but not to interphase cortical arrays or the preprophase band of microtubules. This lack of labelling may be caused by a restriction of antibody access during interphase, but more likely by a cell division-specific conformational change in the gamma-tubulin molecule. Our antibody also gave an organelle-like labelling, not described before, which may represent storage forms or precursors of gamma-tubulin, perhaps related to plastid-based microtubule initiation in hepatics and hornworts.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

Calcium: silver bullet in signaling

Accumulating evidence suggests that Ca(2+) serves as a messenger in many normal growth and developmental process and in plant responses to biotic and abiotic stresses. Numerous signals have been shown to induce transient elevation of [Ca(2+)](cyt) in plants. Genetic, biochemical, molecular and cell biological approaches in recent years have resulted in significant progress in identifying several Ca(2+)-sensing proteins in plants and in understanding the function of some of these Ca(2+)-regulated proteins at the cellular and whole plant level. As more and more Ca(2+)-sensing proteins are identified it is becoming apparent that plants have several unique Ca(2+)-sensing proteins and that the downstream components of Ca(2+) signaling in plants have novel features and regulatory mechanisms. Although the mechanisms by which Ca(2+) regulates diverse biochemical and molecular processes and eventually physiological processes in response to diverse signals are beginning to be understood, recent studies have raised many interesting questions. Despite the fact that Ca(2+) sensing proteins are being identified at a rapid pace, progress on the function(s) of many of them is limited. Studies on plant 'signalome' - the identification of all signaling components in all messengers mediated transduction pathways, analysis of their function and regulation, and cross talk among these components - should help in understanding the inner workings of plant cell responses to diverse signals. New functional genomics approaches such as reverse genetics, microarray analyses coupled with in vivo protein-protein interaction studies and proteomics should not only permit functional analysis of various components in Ca(2+) signaling but also enable identification of a complex network of interactions.

Fine structure analysis of the yeast centrin, Cdc31p, identifies residues specific for cell morphology and spindle pole body duplication

Centrin/Cdc31p is a Ca2+-binding protein related to calmodulin found in the MTOC of diverse organisms. In yeast, Cdc31p localizes to the SPB where it interacts with Kar1p and is required for SPB duplication. Recent findings suggest that centrin also functions elsewhere in the cell. To dissect the functions of Cdc31p, we generated cdc31 mutations chosen only for temperature sensitivity, but otherwise unbiased as to phenotype. Three phenotypes of the cdc31 mutants, temperature sensitivity, G2/M arrest, and cell lysis, were not well correlated, indicating that the mutations may differentially affect Cdc31p's interactions with other proteins. Alleles near the C-terminal region exhibited high G2/M arrest and genetic interactions with kar1-Delta17, suggesting that this region modulates an SPB-related function. Alleles causing high lysis and reduced Kic1p kinase activity mapped to the middle of the gene, suggesting disruption of a KIC1-like function and defects in activating Kic1p. A third region conferred temperature sensitivity without affecting cell lysis or G2/M arrest, suggesting that it defines a third function. Mutations in the C-terminal region were also defective for interaction with Kic1p. Mapping the alleles onto a predicted structure of Cdc31p, we have identified surfaces likely to be important for interacting with both Kar1p and Kic1p.

cDNA isolation, characterization, and protein intracellular localization of a katanin-like p60 subunit from Arabidopsis thaliana

Katanin, a heterodimeric protein with ATP-dependent microtubule-severing activity, localizes to the centrosome in animal cells. Widespread occurrence is suspected as several species contain homologs to the katanin p60 subunit. Recently we isolated an Arabidopsis thaliana cDNA with significant identity to the p60 subunit of sea urchin katanin. Like p60, the encoded protein is a member of the AAA superfamily of ATPases, containing the Walker ATP binding consensus and the signature AAA minimal consensus sequences within a single larger AAA/CAD amino acid motif. Phylogenetic analysis placed the encoded protein in the AAA subfamily of cytoskeleton-interactive proteins, where it formed a strongly supported clade with 4 other members identified as katanin p60 subunits. The clone was named AtKSS (Arabidopsis thaliana katanin-like protein small subunit). Western blots, performed using a polyclonal antibody raised against recombinant AtKSS, revealed AtKSS is present in protein extracts of all Arabidopsis organs examined. To evaluate potential interactions between AtKSS and the cytoskeleton, the intracellular localization of AtKSS was correlated with that of tubulin. AtKSS was found in perinuclear regions during interphase, surrounding the spindle poles during mitosis, but was absent from the preprophase band and phragmoplast microtubule arrays. These data support the thesis that AtKSS is an Arabidopsis homolog of the p60 subunit of katanin. Its cell cycle-dependent distribution is consistent with microtubule-severing activity, but additional studies will better define its role.

Cytokinesis in flowering plants: more than one way to divide a cell

Several different cytokinetic mechanisms operate in flowering plants. During 'conventional' somatic cytokinesis, the mitotic spindle remnants give rise to a phragmoplast that serves as a framework for the assembly of the cell plate. Cell plates fuse with the parental plasma membrane at specific cortical sites previously defined by the preprophase band of microtubules. In nuclear endosperms, meiocytes, and gametophytic cells, cytokinesis occurs without preprophase bands. The position of the new cell walls is determined instead by interacting arrays of microtubules that radiate from the nuclear envelope surfaces. The nuclear cytoplasmic domains defined by these microtubule arrays demarcate the boundaries of the future cells. Recent studies have provided new insights into the ultrastructural similarities and dissimilarities between conventional and non-conventional cytokinesis. Numerous proteins have also been localized to cytokinesis-related cytoskeletal arrays and cell plates but the functions of most of them have yet to be elucidated.

Higher plant cells: gamma-tubulin and microtubule nucleation in the absence of centrosomes

The assembly of the higher plant cytoskeleton poses several fundamental questions. Since different microtubule arrays are successively assembled during the cell cycle in the absence of centrosomes, we can ask how these arrays are assembled and spatially organized. Two hypotheses are under debate. Either multiple nucleation sites are responsible for the assembly and organization of microtubule arrays or microtubule nucleation takes place at one site, the nuclear surface. In the latter case, microtubule nucleation and organization would be two distinct but coregulated processes. During recent years, novel approaches have provided entirely new insights to understand the assembly and dynamics of the plant cytoskeleton. In the present review, we summarize advances made in microscopy and in molecular biology which lead to novel hypotheses and open up new fields of investigation. From the results obtained, it is clear that the higher plant cell is a powerful model system to investigate cytoskeletal organization in acentrosomal eukaryotic cells.